Understanding granulocyte homeostasis is crucial because producing too few granulocytes results in increased risk for infection (neutropenia), while producing too many granulocytes can result in severe tissue damage and death (neutrophilia, myeloproliferative disorders). Current knowledge illustrates many factors that can modify granulocyte versus monocyte production; however, we still do not understand a fundamental question: which specific factors control the granulocyte-monocyte lineage decision during homeostatic production in vivo? To discern the transcriptional network underlying the binary fate decision between neutrophil granulocyte and monocyte lineage decisions as they occur at steady state (homeostatic control), we have performed single-cell RNA-seq on bone-marrow granulocyte-monocyte progenitors (GMP). Our bioinformatic analyses reveal a varied, but coherent spectrum of gene expression patterns in individual murine GMPs. The majority of cells could be clustered into ones expressing either granulocytic or monocytic genes, suggesting that they were primed for lineage determination. A minority of GMPs expressed a mixed-lineage pattern of genes. Deeper analyses of the single-cell data implicate the repression of a key requisite factor for monopoiesis in mice and man (Irf8) by a key requisite factor for granulopoiesis in mice and man (Gfi1) as the central mechanism for homeostatic control of the granulocyte-monocyte lineage decision in vivo. We propose to determine the transcriptional program underlying granulocyte-monocyte lineage fate decisions during homeostasis, and then define the impact of severe congenital neutropenia (SCN)-associated mutations on transcriptional control of granulopoiesis in mice and man. We expect to delineate a cross-species transcriptional signature of granulopoiesis, and define the impact of neutropenic stress.